1. Field of the Invention
This invention relates to a laminated film with gas barrier properties (a gas barrier laminated film), used in the field of the packaging of food, pharmaceuticals and so forth.
2. Description of the Related Art
In recent years, packaging materials used for the packaging of food, medicines and so forth are required to block the effects of oxygen and water vapor passing through the packaging materials or other gases causative of changes in quality of contents, in order to prevent changes in quality of contents, in particular, in the case of food, in order to prevent proteins or fats and oils from being oxidized or undergoing denaturation and also preserve taste or freshness, and, in the case of pharmaceuticals required to be handled in a germless condition, in order to prevent effective components from undergoing changes in quality and maintain their efficacy. Thus, such packaging materials are sought to be endowed with gas barrier properties for shutting off these gases (gaseous matter).
Accordingly, those having been hitherto commonly used as packaging films are Gas barrier laminated films having laminates or coatings formed of resin compositions comprised of a polymer commonly said to have relatively high Gas barrier properties, such as polyvinyl alcohol (hereinafter "PVA"), an ethylene/vinyl acetate copolymer (EVOH) or polyvinylidene chloride resin (hereinafter "PVDC").
Metal deposited films with gas barrier properties are also in wide use, which are formed of suitable polymeric resin compositions (which may be comprised of a resin not having high gas barrier properties by itself) and metals such as aluminum or metal compounds deposited thereon. Nowadays, inorganic compound deposited films have been developed, which comprise a substrate comprised of a polymeric material having a transparency and a light-transmitting thin film such as a silicon oxide (SiO.sub.x) thin film of silicon monoxide (SiO) or the like or a magnesium oxide (MgO) thin film formed on the substrate by a film-forming means such as vacuum deposition. These metal deposited films and inorganic compound deposited films have higher gas barrier properties than the above gas barrier laminated films formed of polymeric resin compositions, may undergo less deterioration in an environment of high humidity, and hence begin to be commonly used as packaging films.
The above gas barrier laminated films making use of a PVA or EVOH type polymeric resin composition, however, have so high temperature dependence and moisture dependence that their gas barrier properties may become poor in an environment of high temperature or high humidity, in particular, their water-vapor barrier properties may become poor. In some instances, depending on how packages are used, the gas barrier properties may become greatly poor when treated by boiling or treated in a retort.
The gas barrier laminated films making use of a PVDC type polymeric resin composition have a small humidity dependence, but have the problem that a high gas barrier material having oxygen barrier properties (oxygen transmission) of not more than 1 cm.sup.3 /m.sup.2.day.atm) can be actually obtained therefrom with difficulty. The PVDC type polymeric resin composition also contains chlorine in a large quantity, and hence has a problem on how to dispose of waste, e.g., on thermal disposal or recycling.
As for the above metal deposited films having a deposit of a metal or metal compound or the inorganic compound deposited films having a silicon oxide thin film of silicon monoxide (SiO) or the like or a magnesium oxide (MgO) thin film formed by deposition, they are laminated materials formed of components very different from each other in properties such as chemical properties and thermal properties, as exemplified by a laminated material comprised of a resin film and a metal deposit and a laminated material comprised of a resin film and an inorganic compound deposit. Hence, there is the problem that no satisfactory adhesion can be achieved between the both.
The inorganic compound deposited thin films used as gas barrier layers are also lacking in flexibility and has a low endurance to deflection and folding. Hence, care must be taken to handle them. In particular, there is the problem that they cause cracks when packaging materials are post-worked for printing, laminating, bag-making and so forth, to make gas barrier properties very poor.
The gas barrier layers such as inorganic compound deposited thin films are formed by a vacuum process such as vacuum deposition, sputtering or plasma-assisted chemical vapor deposition, and there is also the problem that the apparatus used therefor are expensive. Also, when the gas barrier layers are formed by such a vacuum process, the substrate resin film may locally have a high temperature, so that the substrate may be damaged or the substrate may undergo decomposition or gas escape at the part having a low molecular weight or the part containing a plasticizer or the like, to cause defects or pinholes in inorganic thin films serving as the gas barrier layers. Hence, such layers can not achieve high gas barrier properties and also have a problem of a high production cost.
Under such circumstances, a gas barrier material comprised of a substrate and a metal alkoxide coating formed thereon is proposed, as disclosed in Japanese Patent Application Laid-open No. 62-295931. This metal alkoxide coating has a flexibility to a certain degree and also can be produced by liquid-phase coating. Hence, it can promise a low production cost.
The above gas barrier material can be said to have been improved in gas barrier properties compared with an instance where the substrate is used alone, but can not be said to have satisfactory gas barrier properties in practical use.
As a process for producing a resin-coated product endowed with gas barrier properties, Japanese Patent Application Laid-open No. 5-9315 also discloses a process comprising the steps of forming a silicon oxide (SiO.sub.x) deposited thin film on a substrate, and coating thereon a solution comprised of a mixture of SiO.sub.2 particles and a water-soluble resin or water-based emulsion, followed by drying.
According to this production process, the layer formed of the mixture of SiO.sub.2 particles and a resin by its coating on a SiO.sub.x deposited thin film can prevent microcracks from running through the SiO.sub.x deposited film when deformed by an external force to protect the crack portions., so that its gas barrier properties can be prevented from lowering.
However, the barrier material obtained by the above process only prevents microcracks from running through the deposited film to thereby prevent the gas barrier properties from lowering, and is only effective for merely protecting the deposited film. Hence, this material leaves the problem that it can have no higher gas barrier properties than the deposited film and can not achieve any higher gas barrier properties.